A few weeks ago, I went looking for an in-circuit emulator/debugger for my trusty old Apple II and stumbled across a project called "ICE-T65"
http://www.stardot.org.uk/forums/viewtopic.php?f=44&t=9655
The hardware is a small FPGA mounted on a 40-pin board that has a similar form-factor to a 6502 processor, and with the right software, can be used as a pin-compatible, plug-in replacement for a 6502. Installation is as simple as popping out the CPU and putting this board in it's place.
My goal then became to add a virtual Disk II interface into the FPGA code. Initially, I was thinking of using an SD card for backing store, but then decided it would be more flexible if I added a network interface and allowed it to boot disk images over the network. (i.e. you can use this to boot a real physical Apple II from a disk image hosted on a web site)